Fix wrong range check

src/vm/gfx.h

@@ -209,7 +209,7 @@ namespace retro8
 
     public:
       Font() { }
-      inline const sequential_sprite_t* glyph(char c) const { return c < 128 ? &glyphs[c] : nullptr; }
+      inline const sequential_sprite_t* glyph(char c) const { return (unsigned) c < 128 ? &glyphs[(unsigned) c] : nullptr; }
       inline const sequential_sprite_t* specialGlyph(size_t i) const { return &glyphs[128+i]; }
 
       void load();
@@ -221,4 +221,4 @@ namespace retro8
   };
 
 
-}
\ No newline at end of file
+}